1. Field of the Invention
The present invention relates to air-fuel ratio control apparatus and method for an internal combustion engine and, more particularly, relates to a technique for improving a control accuracy of an air-fuel ratio when an EGR (Exhaust Gas Recirculation) control is used together with a normal air-fuel ratio feedback control such as an intake-air quantity control.
2. Description of the Related Art
A Japanese Patent Application First Publication No. Showa 63-94061 published on Apr. 25, 1988 exemplifies a previously proposed air-fuel ratio control apparatus in which Exhaust Gas Recirculation (EGR) control valve is disposed within an EGR passage which links between an intake-air passage and an exhaust gas passage for controlling an EGR gas quantity so that an opening degree of the EGR valve or a valve opening rate thereof is feed-back controlled to make the air-fuel ratio substantially equal to a target air-fuel ratio (or so called, a stoichiometric air-fuel ratio).
That is to say, in the same engine driving condition, an opening degree of the EGR valve is modified to increase or decrease the EGR gas quantity. At this time, a fresh air quantity is decreased or increased by a quantity corresponding to the modification of the opening degree of the EGR valve so that the air-fuel ratio can be modified. Thus, the air-fuel ratio is controlled to be made substantially equal to the target air-fuel ratio. In a case where an intake-air quantity (the fresh air quantity) is directly controlled to achieve the air-fuel ratio feedback control, the fresh air quantity is caused to be varied. At this time, an EGR quantity is accordingly varied so as to satisfy a target EGR rate. Thus, the fresh air quantity is, in turn, varied. Consequently, the air-fuel ratio is deviated from the target air-fuel ratio. Therefore, a delay occurs in a convergence of the air-fuel ratio into the target air-fuel ratio. However, when the air-fuel ratio feedback control is performed through the EGR control, a quick convergence of the air-fuel ratio into the target air-fuel ratio can be achieved since no re-adjustment for the EGR rate is needed.
However, if a control over the opening degree of the EGR valve based on the air-fuel ratio varied due to an intake-air ripple or engine rotation variation is performed, the opening degree of the EGR valve is varied and the EGR gas quantity is accordingly varied. Consequently, a divergence of the air-fuel ratio from the target air-fuel ratio may occur.
To avoid such a divergence of the air-fuel ratio as described above, another air-fuel ratio control apparatus has previously been proposed according to a Japanese Patent Application First Publication No. Heisei 10-220304 published on Aug. 18, 1998. In the above-described other previously proposed air-fuel ratio control apparatus, the opening degree of the EGR valve is controlled on the basis of a smoothed air-fuel ratio obtained by smoothing the detected air-fuel ratio. Thus, the variation in the opening degree of the EGR valve is reduced. In addition, the smoothed air-fuel ratio is calculated using a smoothing coefficient defined according to the opening degree of the EGR valve. The opening degree of the EGR valve is controlled on the basis of the smoothed air-fuel ratio. Hence, while a response characteristic of the opening degree of the EGR valve is secured, the air-fuel ratio is made substantially equal to the target air-fuel ratio with a favorable convergence characteristic.
However, since, in the air-fuel ratio feedback control performed through the EGR control as in the former previously proposed air-fuel ratio control apparatus, a change rate of an EGR controlled variable used for the air-fuel ratio feedback control to the EGR rate, in a small region of the EGR rate, is relatively large, the air-fuel ratio feedback control through the EGR control cannot achieve a control of the EGR rate to an appropriate EGR rate.
In addition, although some other previously proposed air-fuel ratio control apparatuses perform the air-fuel ratio feedback control through the EGR control so as to maintain a relationship between the EGR rate and air-fuel ratio (air excess factor) constant, the constant relationship cannot be established over a whole control region. Hence, if, for example, the air-fuel ratio feedback control is performed through the EGR control to achieve the target air-fuel ratio (a target air excess factor) during an acceleration including a moderate acceleration, a target value of the EGR rate cannot be achieved.
Furthermore, in a still another previously proposed air-fuel ratio control apparatus in which a rich spike control such that the air-fuel ratio is controlled to be rich in order to reduce NOx adsorbed onto an NOx trap catalyst by HC in the exhaust gas to regenerate the catalyst, the NOx trap catalyst being disposed to adsorb NOx thereonto is carried out, it is necessary to control accurately the EGR rate and air-fuel ratio to satisfy both of a stability of fuel combustion and an exhaust gas purification performance during an execution of the rich spike control. However, if the air-fuel ratio feedback control is performed through the EGR control, the EGR controlled variable becomes excessively large or, on the contrary, becomes excessively small so that an instability of fuel combustion and a reduction in the exhaust gas purification may occur.
It is, therefore, an object of the present invention to provide improved air-fuel ratio control apparatus and method for an internal combustion engine which can effectively execute the air-fuel ratio feedback control through the EGR control so as to achieve highly accurate EGR control and air-fuel ratio control with favorable response and convergence characteristics while avoiding possibilities of occurrences in both of the instability of fuel combustion and the reduced exhaust gas purification performance.
According to one aspect of the present invention, there is provided with an air-fuel ratio control apparatus for an internal combustion engine having an EGR valve interposed in an EGR passage between an intake manifold and an exhaust manifold, comprising: a target EGR quantity calculating section that calculates a target EGR quantity; a control execution determining section that determines which of air-fuel ratio feedback controls through an EGR control and through an intake-air quantity is to be executed according to the target EGR quantity; and a control execution section that selectively executes one of the air-fuel ratio feedback controls according to a result of a determination by the control execution determining section.
According to another aspect of the present invention, there is provided air-fuel ratio control method for an internal combustion engine having an EGR valve interposed in an EGR passage between an intake manifold and an exhaust manifold, comprising: calculating a target EGR quantity; determining which of air-fuel ratio feedback controls through an EGR control and through an intake-air quantity is to be executed according to the target EGR quantity; and selectively executing one of the air-fuel ratio feedback controls according to a result of a determination of which of the air-fuel feedback controls is to be executed.
This summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features.